Shadow mask having focusing function and method of making same

ABSTRACT

A METHOD OF MAKING A SHADOW MASK HAVING A FOCUSING FUNCTION COMPRISING THE STEPS OF PREPARING AN INSULATOR HAVING THE FORM OF A PASTE PREPARED BY MIXING POWDERED GLASS IN A DISPERSION MEDIUM, THINLY COATING THE INSULATOR ON A SHADOW MASK ELECTRODE HAVING ELECTRON BEAM HOLES, SUBJECTING THE INSULATOR TO HEAT TREATMENT, AND DISPOSING A SHADOW MASK LENS ELECTRODE ON THE INSULATOR.

TADAO OKABE ETAL 3,668,002 SHADOW MASK HAVING FOCUSING FUNCTION June 6, 1972 AND METHOD OF MAKING SAME Filed June 25, 1969 H6; PRIOR ART Fla 3 FIG. 4

FIG. 215

FIG. 2a

INVENTOR5 SHOZO TAMI-(RA,

T'ADAO OKABE MAK TO TANA A MAsnK/Izu FHKUSHIMA and MITsuRu OIkAwA ATTORNEYS United States Patent O US. Cl. 117-210 6 Claims ABSTRACT OF THE DISCLOSURE A method of making a shadow mask having a focusing function comprising the steps of preparing an insulator having the form of a paste prepared by mixing powdered glass in a dispersion medium, thinly coating the insulator on a shadow mask electrode having electron beam holes, subjecting the insulator to heat treatment, and disposing a shadow mask lens electrode on the insulator.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to methods of making a shadow mask having a focusing function and more particularly to a method of making a shadow mask having a focusing function in which a shadow mask electrode having holes for passage therethrough of electron beams is disposed opposite to a shadow mask lens electrode with an insulator interposed therebetween and a slight potential difference is applied between these two electrodes to form an electron lens in each of the holes.

Description of the prior art US. Patent application Ser. No. 622,996 now Pat. 3,586,900 discloses a shadow mask having a focusing function in which a shadow mask electrode having holes for passage of electron beams therethrough is disposed opposite to a shadow mask lens electrode (which will be described later) and a slight potential difference is applied between these two electrodes to form an electron lens in each of the holes. It is possible by employing a shadow mask having such a structure to enhance the beam utility factor, obtain a brighter picture and remarkably improve the color purity and resolution.

In the manufacture of such a shadow mask, the shadow mask lens electrode must be supported on the shadow mask electrode through a layer of an insulator. However, in a shadow mask which is so sized that the holes for the passage of electron beams have a diameter of the order of 0.5 mm. and the spacing between the two electrodes is of the order of 0.25 mm., it is extremely difiicult to form an insulating layer on the shadow mask electrode without clogging the numerous electron beam holes in the shadow mask. The shadow mask is also encountered with a problem that the insulating layer may be charged to adversely affect the focusing function for the electron beams. Because of the above fact, the shadow mask of this kind has not yet been put into practical use in spite of many excellent advantages possessed thereby.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of making a shadow mask having a focusing function which includes an improved step of easily forming an insulating layer on a shadow mask electrode without clogging numerous electron beam holes in the shadow mask electrode.

3,668,002 Patented June 6, 1972 Another object of the present invention is to provide a shadow mask having a focusing function which is so constructed as to prevent charging of the insulating layer interposed between the shadow mask electrode and a shadow mask lens electrode.

In order to attain the above objects, the method according to the present invention comprises the steps of coating an insulator having the form of a paste prepared by mixing powdered glass in a dispersion medium on a shadow mask electrode, subjecting the insulator to heat treatment, and providing a shadow mask lens electrode on the insulating layer thus formed.

The above and other objects, features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view of part of a color cathode ray tube equipped with a prior art shadow mask having a focusing function.

FIGS. 2a, 2b, 3 and 4 are schematic sectional views of part of a shadow mask showing successive steps of manufacture in an embodiment of the present invention.

FIG. 5 is a schematic sectional view of part of a shadow mask showing a practical embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a color cathode ray tube having a prior art shadow mask is provided with a shadow mask electrode 1, a shadow mask lens electrode 2 disposed opposite to the shadow mask electrode 1 with an insulating layer 3 interposed therebetween, a glass face plate 4 of the cathode ray tube, a triad type fluorescent screen 5, a metal back, that is, anode electrode 6, and holes 8 bored in the shadow mask for allowing passage of electron beams 7 therethrough.

A voltage equal to or close to the voltage applied to the anode electrode 6 is applied to the shadow mask lens electrode 2, and a voltage slightly lower (for example, by 1.3 kilovolts) than the voltage applied to the shadow mask lens electrode 2 is applied to the shadow mask electrode 1 so as to generate lines of electric force shown by the arrow in the electron beam holes 8 thereby to form an electron lens in each of the holes 8. The electron beams 7 passing through each hole 8 having such an electric field therein are focused toward the central axis of the hole 8 by the action of the electron lens and impinge against the fluorescent screen 5. By virtue of the focusing function described above, the electron beam spot formed by impingement of the electron beams has an area which is considerably smaller than the area of the hole 8. The shadow mask lens electrode 2 may be a thin film electrode of aluminum or mesh electrode of metal, or an electrode having a plurality of small holes, or such an electrode which has holes whose diameter is substantially equal to or larger than the diameter of the holes of the shadow mask electrode 1 and has portions overlapped with projected images of the respective holes 8 of the shadow mask electrode 1, each of the portions of the shadow mask lens electrode 1 having a conductive portion at least in the center thereof. It will thus be understood that the shadow mask lens electrode 2 may have any suitable structure provided that the lines of electric force as shown by the arrow in FIG. 1 are generated in each hole 8.

While the structure of a shadow mask having a focus ing function has been described in the above, the method of making a shadow mask according to the present invention and the structure of the shadow mask made thereby will now be described in detail.

FIGS. 2a and 2b are sectional views showing a state in which an insulator in the form of a paste prepared by mixing powdered glass in a dispersion medium is coated on a shadow mask electrode 21. Reference numeral 22 designates an insulating layer in the form of a paste thus coated. Reference numeral 23 designates holes 23 for allowing the passage of electron beams therethrough. The paste may, for example, be prepared by mixing 1() grams of powdered glass in 15 cc. of ethanol and thoroughly agitating the mixture. The paste may be applied by means such as a brush or air spray to one surface of the shadow mask electrode 21, especially to that surface in which the electron beam holes 23 have a diverging end so that the insulating layer 22 is provided on that specific surface. Depending on the factors including the size of the holes 23 in the shadow mask electrode 21, desired thickness of the insulating layer 22 to be coated and viscosity of the paste, the pasty insulating layer 22 may be coated in such a state that the insulating layer 22 has already openings corresponding to the electron beam holes 23 as shown in FIG. 2a or the insulating layer 22 does not have any such openings as shown in FIG. 2b. FIG. 2a represents a case in which the pasty insulating layer 22 is very thinly coated, while FIG. 2b represents a case in which the pasty insulating layer 22 is slightly thickly coated. In this latter case, it is necessary to coat the insulating layer 22 with such a thickness that those portions corresponding to the holes 23 in the shadow mask electrode 21 are opened in the later baking step due to the surface tension of the molten glass itself.

The shadow mask electrode 21 coated with the insulating layer 22 of powdered glass is then dried for about minutes with dry air and is heated for about 10 to 60 minutes in an electric furnace regulated at a temperature of about 500 C. By this treatment, the powdered glass is fused to attach to the shadow mask electrode 21 and the glass material existing in the electron beam holes 23 moves away from the holes 23 as shown in FIG. 3 due to the surface tension of the molten glass, with the result -.that the rising insulating layer 22 of glass is formed on one surface of the shadow mask electrode 21.

The potential difference established between the shadow mask electrode 21 and the shadow mask lens electrode is of the order of 1' kilovolt as described above and the thickness of the glass insulating layer 22 formed in the above manner is of the order of 0.1 mm. The glass insulating layer 22 having such a small thickness is sufficiently adequate since such glass material has an extremely high breakdown voltage. The glass material preferably used to form the glass insulating layer 22 is an amorphous and inorganic material which has a high specific resistance and whose fusing point is less than a temperature at which the shadow mask electrode will be thermally deformed, or less than about 1000 C. For example, the glass material may be a silicate containing lead oxide, boron oxide or zinc oxide in a large amount, or a substance such as selenium, arsenic, gallium or sulfur, or a mixture of these substances. Since the breakdown voltage varies widely depending on the glass material employed, it is necessary to determine the thickness of the glass insulating layer so that the insulating layer of specific material has a sufficiently large breakdown voltage.

A thick insulating layer for obtaining a required breakdown voltage can easily be realized by repeating the above procedure a plurality of times so as to obtain an insulating layer of the desired thickness. In repeating the above procedure a plurality of times, glass of the same material or glasses of different materials may be employed as desired. In the latter case, it is preferable that glass having a higher fusing point is coated at first and glasses having successively lower fusing points are then coated. This procedure is very convenient in that the insulating layer previously coated is not fused to melt during the baking treatment of the insulating layer coated later. In this connection, it will be apparent that, when glass of the same material is repeatedly coated, powdered glass coated on the previously coated insulating layer is fused to melt first during the baking treatment because the insulating layer coated later is in the form of a paste of powdered glass whereas the insulating layer previously coated is in the form of a sheet of glass. Therefore, by suitably selecting the baking temperature, an insulating layer of the same material as the previously coated insulating layer can be coated on the latter layer without fusing to melt the previously coated insulating layer.

The electron beam hole 23 in the insulating layer 22 formed in the above manner has a conical shape having a small beam inlet and a large beam outlet so that the electrons can freely pass through the hole 23 without impinging against the insulating layer 22. Therefore, the insulating layer 22 is free from being charged and would not adversely affect the focusing of electron beams. When it is found in the course of manufacture that the electron beam holes are clogged or are not shaped to the desired form, a solution of hydrofluoric acid (HF), ammonium fluoride (NH F) or nitric acid (HNO or a mixture of these solutions may be used to treat the holes.

After forming the insulating layer 22 on the shadow mask electrode 21 in the manner described above, a shadow mask lens electrode 24 is disposed in close contact with the insulating layer 22 as shown in FIG. 4. The shadow mask lens electrode 24 may be directly bonded to the insulating layer 22 or may merely be fixed to the frame of the shadow mask so that it may sufiiciently be supported on the shadow mask While being brought in contact with the insulating layer 22.

It will be understood from the foregoing description vw'th regard to the method of making a shadow mask having a focusing function that the electron beam holes 23 in the shadow mask thus made have a conical shape as described above and the electrons can freely pass there through without impinging against the insulating layer 22. However, this is limited to a case in which the electron beam holes 23 are ideally formed under very good conditions. Formation of an insulating layer 22 having ideally conical electron beam holes on the shadow mask electrode having a multiplicity of small beam holes is extremely difiicult, and a part of the insulating layer 22 is commonly exposed in the electron beam holes 23. In such a case, the electrons passing through the electron beam hole 23 attach to the portion of the insulating layer 22 which is exposed in the electron beam hole 23 or the portion of the insulating layer 22 which can be viewed in the axial direction of the hole, thereby to negatively charge the insulating layer 22. In some cases, the electrons at high speed may impinge against the insulating layer 22 to emit secondary electrons thereby to positively charge the insulating layer 22. In either case, such a charge would extremely obstruct the focusing action of the electron lens and the important effect of the shadow mask of this kind would thereby be lost.

FIG. 5 is a sectional view of part of a shadow mask showing an embodiment of the present invention for preventing such an objectionable charging phenomenon as described above. The shadow mask comprises a shadow mask electrode 21, an insulating layer 22, electron beam holes 23, a shadow mask lens electrode 24 and a conductive thin layer 25. The present invention is characterized by the provision of the conductive thin layer 25. The conductive thin layer 25 is formed at least on that portion of the insulating layer 22 which is exposed in the electron beam hole 23 when viewed in the axial direction of the hole. It has been found experimentally that the provision of the conductive thin layer 25 at such a position lends itself to sufiicient focusing of electron beams, reduces astigmatism and ensures a picture of good quality. The conductive thin layer 25 may be of a metal such as aluminum, silver, gold or any other suitable metal. However, aluminum is considered most effective because it is inexpensive and has less tendency to disperse the electrons.

The conductive thin layer 25 may be formed by a method of, for example, metal evaporation after the completion of the baking step shown in FIG. '3. A metal such as aluminum may be evaporated from the side of the shadow mask electrode 21 opposite to the insulating layer 22 so as to easily form the aluminum coating 25 on the exposed portion of the insulating layer 22.

It is probable that the formation of the conductive thin layer 25 may result in a partial short-circuit between the shadow mask electrode 21 and the shadow mask lens electrode 24, or even when such short-circuitin-g does not occur, in a discharge through the conductive thin layer 25 when a slight potential difference is applied between the two electrodes for providing the lens action. Therefore, it is recommended that, after the formation of the conductive thin layer 25, a voltage higher than the voltage for providing the lens action should be applied across the two electrodes so as to melt away and remove any unnecessary portion of the conductive thin layer 25.

From the foregoing description it will be appreciated that a shadow mask having an electron lens in each of the electron beam holes can simply and easily be manufactured by the present invention. It will be appreciated further that the shadow mask made according to such a method has a conductive thin layer formed on that portion of an insulating layer which is exposed in the electron beam hole, and the conductive thin layer acts to remove or minimize charges which obstruct the focusing of electron beams and gives rise to astigmatism. Thus, the effect of a shadow mask having a focusing function can fully be exhibited and a picture of good quality can be reproduced.

The embodiments described in detail above are merely illustrative of the present invention and many changes and modifications may be made without departing from the spirit of the present invention.

We claim:

1. A method of making a shadow mask having a focusing function comprising the steps of preparing a paste by mixing powder of vitreous inorganic material having a high specific resistance in a dispersion medium, coating said paste on at least one surface of a shadow mask electrode to such a thickness that said vitreous material may not clog electron beam holes in said shadow mask electrode after the later step of baking, baking the coating of said paste at a temperature at which said vitreous material is fused to melt thereby forming on said shadow mask electrode an insulating layer having openings at those portions corresponding to said electron beam holes, and disposing a shadow mask lens electrode opposite to said shadow mask electrode with said insulating layer interposed therebetween.

2. A method of making a shadow mask as claimed in claim 1, in which said step of coating the paste of said vitreous material and said step of baking are repeated a plurality of times.

3. A method of making a shadow mask as claimed in claim 2, in which vitreous materials having different fusing points are applied one after another.

4. A method of making a shadow mask as claimed in claim 1, further comprising the step of evaporating a metal from the side of said shadow mask electrode opposite to said insulating layer after said step of baking.

5. A method of making a shadow mask as claimed in claim 1, further comprising the step of forming a conductive thin layer on at least that portion of said insulating layer which is exposed in said electron beam holes when viewed in the axial direction of said electron beam holes.

6. A method of making a shadow mask as claimed in claim 5, wherein said conductive thin layer is a thin layer of aluminum.

References Cited UNITED STATES PATENTS 3,398,309 8/ 1968 Kaplan 3l3--85 WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R. 

